The invention relates to an oscillator intended to provide an output signal having a frequency which is variable as a function of a tuning voltage, comprising:
a passive part having two output terminals between which the output signal is taken, and an input terminal intended to receive the tuning voltage, and comprising two series-arranged variable capacitances, thus forming a capacitive branch whose extremities constitute the output terminals, and of which a junction point between the variable capacitances constitutes the input terminal, the output terminals being connected to a first power supply terminal via two inductances having equal nominal values, PA1 an active part comprising a first and a second transistor each having a biasing terminal, a reference terminal and an output terminal, the output terminals of the first and second transistors being connected to the output terminals of the passive part, the reference terminals of said transistors being connected to a second power supply terminal via a current source, the biasing terminals of the first and second transistors being connected to the output terminals of the second and first transistors, respectively, via two coupling capacitances having equal nominal values.
Such an oscillator is known from the article "A Monolithic GaAs IC for Heterodyne Generation of RF Signals" by Mr Rory L. van Tuyl, published in IEEE Transactions on Electron Devices, vol. ED-28, No. 2. The transistors in this oscillator are realized in gallium arsenide, which gives them the advantage of rapidity. However, these transistors generate a considerable low-frequency noise, which is a major drawback in radiotelephone applications in which the frequency of the output signal of the oscillator is used for selecting a radio-electric signal from a range of given frequencies. The signal generated by the oscillator must have high spectral purity, which cannot be obtained by means of gallium arsenide transistors.
A larger spectral purity may be obtained by using bipolar transistors connected as differential pairs in accordance with the description in the opening paragraph. When used in a portable telephone of, for example the GSM type, such an oscillator receives a regulated power supply voltage elaborated on the basis of a voltage which is present at the terminals of a battery included in this telephone. This regulated voltage has, by nature, low-frequency variations which may reach in the passive part of the oscillator and alter the spectral purity of the signal generated by this oscillator.